The present invention relates generally to processes for the cryogenic distillation of air, and in particular to such processes used to produce at least a nitrogen-enriched vapor product.
Nitrogen is one of the most important industrial gases. A common way to supply nitrogen to a process or a customer is a customer station. Typically, liquid nitrogen is hauled in a tanker from a cryogenic air separation plant or a liquefier to the customer""s site, stored in a tank, optionally pumped to a desired pressure, and vaporized in an ambient vaporizer. This process is thermodynamically very inefficient. However, the equipment is inexpensive and reliable.
Another common process to produce nitrogen on a customer""s site is a cryogenic air separation unit. Air is purified to remove water, CO2, N2O1 and other contaminants that may freeze in a cryogenic distillation column, cooled in a heat exchanger to close to its cryogenic saturation temperature (a temperature at which it starts liquefying after the bulk of contaminants is removed), and separated in a cryogenic distillation column into a nitrogen product and an oxygen-rich product. Cooling takes place against returning product streams. This process is thermodynamically very efficient but the equipment is expensive. Refrigeration is supplied by isentropic expansion of one of the streams in a turbine, or, as a less expensive alternative, by liquid nitrogen injection. Liquid nitrogen injection requires hauling liquid nitrogen to the site and storing the liquid nitrogen in a tank. A customer station is usually required as a backup system.
xe2x80x9cCryogenic saturationxe2x80x9d refers to the state of a gas when, if cooled, a portion of the gas is converted to a liquid. This liquid comprises the major components contained in the cryogenically saturated gas. This is different than ambient saturation, in which the resultant liquid comprises the minor components and/or impurities contained in the vapor.
A xe2x80x9ccryogenxe2x80x9d refers to a liquid that normally exists at xe2x80x9ccryogenic temperatures,xe2x80x9d which are defined as temperatures below xe2x88x92110xc2x0 F.
U.S. Pat. No. 6,202,422 (Brugerolle) discloses an air separation unit integrated with a gas turbine. This patent discloses a nitrogen wash column wherein liquid nitrogen is pumped to the top of the column and air from a gas turbine compressor is purified to remove water, CO2, and other contaminants that may freeze in a cryogenic distillation column. The purified air is cooled to a temperature close to its cryogenic saturation temperature, and is then introduced to the bottom of the column. Air from the gas turbine compressor is at a relatively high pressure, which reduces purification equipment cost. Gaseous nitrogen product is recovered from the top of the column, warmed against a feed air stream, and subsequently used in the gas turbine.
U.S. Pat. No. 6,276,171 (Brugerolle) and WO 00/60294 (Brugerolle) disclose a nitrogen wash column integrated with an air separation unit. Air to the column may come from a separate compressor. The air is purified by removing water, CO2 and other contaminants that may freeze in a cryogenic distillation column, and the purified air is cooled against a nitrogen product in a separate heat exchanger. The purposes of the system and process are: 1) to increase oxygen and nitrogen production of the air separation unit, and 2) to be able to operate the air separation unit and the nitrogen wash column independently of one another. For example, when the air separation unit is down, liquid nitrogen to the nitrogen wash column comes from a tank. Oxygen-rich liquid can be stored in another tank and returned to the air separation unit when it is back on line. Separate heat exchangers, compressors, and air purifiers help accomplish this task. This process is a variation of the thermodynamically efficient cryogenic air separation process discussed previously.
There are many methods commonly used in the industry to purify air fed to an air separation unit such as a nitrogen wash column. One is a molecular sieve or activated alumina adsorber unit, which adsorbs water, CO2, N2O, and other contaminants that may freeze in the heat exchanger. It requires a low-pressure gas stream for regeneration. Another method is a reversing heat exchanger or a regenerator. Contaminants freeze out in a heat exchanger that cools incoming air from close-to-ambient temperature to close-to-cryogenic saturation temperature by exchanging heat with cryogenic vapor product or products. One unit is on stream while another is being regenerated. An adsorber unit with or without a heat exchanger, or a reversing heat exchanger, is expensive.
It is desired to have an improved process for the production of a nitrogen-enriched vapor product.
It is further desired to have a more efficient process for the production of a nitrogen-enriched vapor product.
It is still further desired to have a more efficient and improved process for the production of a nitrogen-enriched vapor product which overcomes the difficulties and disadvantages of the prior art processes to provide better and more advantageous results.
The invention is a process and a system for producing a nitrogen-enriched vapor product from a supply of a nitrogen-rich liquid. There are several variations of the process and several variations of the system.
The process, which uses a purifying device and a distillation column having a distillation zone, includes multiple steps. The first step is to feed at least a portion of the supply of the nitrogen-rich liquid to the distillation zone at a first location. The second step is to feed a stream of a gas containing nitrogen and at least one contaminant to the purifying device, wherein the gas is cooled by a cryogenic liquid whereby at least a portion of the at least one contaminant condenses, solidifies, or dissolves. The third step is to eventually feed at least a portion of the cool gas from the purifying device to the distillation zone at a second location below the first location. The fourth step is to withdraw a stream of the nitrogen-enriched vapor product from the distillation zone. The fifth step is to withdraw a stream of an oxygen-enriched liquid from the distillation zone.
In one variation of the process, at least a portion of the cryogenic liquid is at least a portion of the stream of the oxygen-enriched liquid. In another variation, the purifying device is located inside the distillation column, while in another variation, the purifying device is located outside the distillation column. In yet another variation, the gas containing nitrogen comprises air, while in another variation, the gas containing nitrogen has a composition different than a composition of atmospheric air.
The system for producing a nitrogen-enriched vapor product from a supply of a nitrogen-rich liquid includes multiple elements. The first element is a means for containing the supply of the nitrogen-rich liquid. The second element is a distillation column having a distillation zone inside the distillation column. The second element is a purifying device in fluid communication with the distillation column. The fourth element is a means for feeding at least a portion of the supply of the nitrogen-rich liquid to the distillation zone at a first location. The fifth element is a supply of a gas containing nitrogen and at least one contaminant. The sixth element is a means for eventually feeding a stream of the supply of the gas to the purifying device, wherein the gas is cooled by a cryogenic liquid whereby at least a portion of the at least one contaminant condenses, solidifies, or dissolves. The seventh element is a means for withdrawing a stream of the nitrogen-enriched vapor product from the distillation zone. The eighth element is a means for withdrawing a stream of an oxygen-enriched liquid from the distillation zone.
In one variation of the system, at least a portion of the cryogenic liquid is at least a portion of the stream of the oxygen-enriched liquid. In another variation, the purifying device is located inside the distillation column, while in another variation, the purifying device is located outside the distillation column. In yet another variation, the gas containing nitrogen comprises air, while in another variation, the gas containing nitrogen has a composition different than a composition of atmospheric air.